Immunological ignorance occurs if an antigen remains outside secondary lymphoid tissues, the site where mature antigen presenting cells (ARC) activate naive T and B lymphocytes. Although ignorance is an important mechanism by which autoreactive T cells avoid self-antigens, ignorance of transplanted organs is seldom observed. Even allografts that are allowed to recover from the surgical procedure continue to present antigen in the draining lymphoid tissue and undergo either acute or chronic rejection when immunosuppression is withdrawn or upon the transfer of exogenous naive or effector/memory T cells. This suggests that transplantation poses a barrier to immunologic ignorance even after long periods of stable allograft function. Exploring these barriers could provide new strategies that facilitate allograft acceptance and prevent chronic rejection. Therefore, we propose in this component of the PPG to investigate why the immune system does not ignore a 'healed'allograft. The specific aims will focus on two hypotheses that integrate with Projects 1 and 2 of the PPG, respectively: (1) Innate immune activation persists in a "healed" allograft leading to APC maturation, naive T cell activation, and enhanced entry of effector/memory T cells into the graft;and (2) lymphodepletion that often accompanies tolerance-inducing regimens is responsible for continuous recognition of the graft by naive T cells that undergo lymphopenia-induced proliferation (LIP) and transform into memory-like lymphocytes. In specific aim 1, we will investigate what perpetuates the innate immune response to a 'healed'allograft, with emphasis on innate lymphocytes (NK and iNKT cells), neutrophils, the complement system, and direct activation of APCs via TLR-signaling pathways. In specific aim 2, we will investigate the mechanisms responsible for regulating lymphophenia-induced proliferation of naTve T cells, with emphasis on CTLA-4 and TGFb. Understanding these mechanisms would allow us to develop ignorance-based strategies that facilitate allograft acceptance and prevent chronic rejection. This Project is critically dependent on the Histopathology Core for processing and analysis of cardiac allograft samples, quantitating allograft vasculopathy, and phenotyping dendritic cells.